In multicellular organisms the establishment, maintenance, and programmed alterations of cell-type specific gene expression patterns are regulated by epigenetic mechanisms. Thus, epigenetic alterations (DNA methylation, DNA associated Polycomb-Trithorax protein complexes, histone modifications) ensure the unique transcriptional activity and phenotypic diversity of diploid cells that carry identical or nearly identical DNA sequences.

Because DNA methyltransferase I (DNMT1) associates with replication foci during S phase and prefers hemimethylated DNA as a substrate, DNMT1 ensures the...